Transcranial Magnetic Stimulation (TMS) has been employed to treat neurological and psychiatric illnesses, including depression refractory to the administration of drugs. The number of treatable conditions may significantly increase as the depth of the target increases. Systems for targeting neural structures at depth (e.g., Schneider and Mishelevich, U.S. patent application Ser. No. 10/821,807, and Mishelevich and Schneider, U.S. patent application Ser. No. 11/429,504) may include multiple electromagnets, the firing of which must be coordinated. TMS stimulation of deep targets would potentially permit treatment of a variety of conditions such as chronic pain, addiction, obesity, depression, Alzheimer's disease, and Parkinson's disease. Conventional rTMS (repetitive Transcranial Magnetic Stimulation) is capable of effectively stimulating only the outer cortical layer of the brain, and treats depression indirectly, by stimulating neural pathways that run from the prefrontal cortical surface to the cingulate gyrus, rather than hitting the target directly. It is preferable to stimulate deep structures such as the cingulate gyrus directly, but when targeting deep neural structures with rTMS, care must be taken to avoid over-stimulating superficial structures to eliminate undesired side effects such as seizures or producing unintended neural-stimulation results. It is thus necessary to avoid having successive pulses from the same electromagnet passing through such superficial structures while targeting the deep structure, particularly when the strength of the magnetic field (e.g., the field intensity) is sufficient to activate or depolarize non-target tissues between the TMS electromagnet and the deep brain target.
To effectively elicit an action potential in a neural structure such as a deep brain target, adequate stimulation must be received in a time period that is less than the minimum time (usually expressed as chronaxie) that it takes the target neural membrane to re-polarize. Otherwise threshold for generating an action potential will not be achieved. With respect to another time scale, for a given neural structure, stimulating pulses must be received within a maximum effective time interval such that the effect of the generated action potentials is additive. Neural elements are typically highly interconnected and the actual final target element to be stimulated will receive inputs from multiple sources.
Typical deep brain target regions may include, for example, the insula and the cingulate gyrus. Stimulation of a target deep brain regions without stimulating or depressing stimulation of nearby non-target brain region, and particularly brain regions between the target deep brain region and the TMS electromagnet, may be achieved by optimizing the power applied to the TMS electromagnet(s) so that the electromagnetic field(s) reaching the target sum to achieve the desired stimulation. Optimization typically means minimizing the power applied (and/or the rate power is applied) to a TMS electromagnet so that the intervening non-target regions are not stimulated.
Thus, power applied to any given electromagnet, and/or the rate that the power is applied, is preferably limited. However, the power applied by one or more TMS electromagnets intended to stimulate a deep brain target such as the insula or cingulate gyrus must be sufficient to activate the deep brain target. While limiting the power and frequency from a single stimulating location may protect structures superficial to the deeper target, it may be impossible to effectively stimulate a deep target because of the rapid fall off of the magnetic field. The attenuation of the magnetic field is commonly believed to be equivalent to roughly 1/(distance)2 at short distances. This inverse-square relationship is particularly significant, and a version of this relationship has been used to determine the strength needed for stimulation of a deep brain target region by one or more TMS electromagnets.
Known deep-brain stimulation techniques, including those described by Mishelevich and Schneider described above, have generally applied the inverse-square relationship to determine the stimulation power and/or frequency to be applied. Described herein are methods of more accurately estimating the applied power necessary from one or more TMS electromagnets to prevent or minimize stimulation of intervening non-target regions. In particular, the methods described herein may be used to determine the power (e.g., minimum power) necessary to stimulate a deep brain region target. This may allow the stimulation to be kept below the motor threshold (MT) for stimulation, to avoid stimulation of the intervening non-target region.
In general, the treatment of specific neurological and psychiatric illnesses using rTMS requires that specific neuroanatomical structures be targeted using specific pulse parameters. This may be greatly facilitated by using magnetic coils placed in specific positions with respect to one another, and with respect to the neuroanatomical target. Proper coil configuration ensures that neuromodulation of the targeted structure is optimally accomplished, with minimal perturbation of neural tissue between the electromagnet coils and the target, including nearby areas outside of the target region. Described herein are methods and systems for the optimal placement of TMS coils and coil arrays relative to the human head, specific geometries, and specific brain targets, as well as method and systems for determining the optimal power and/or frequency applied to stimulate the target without stimulating non-target regions.
This application hereby incorporates by reference in their entirety the following co-pending patent applications: “DEVICE AND METHOD FOR TREATING HYPERTENSION VIA NON-INVASIVE NEUROMODULATION” (PCT/US2008/071663), filed Jul. 30, 2008; “GANTRY AND SWITCHES FOR POSITION-BASED TRIGGERING OF TMS PULSES IN MOVING COILS” (PCT/US2008/072930), filed Aug. 12, 2008; “FIRING PATTERNS FOR DEEP BRAIN TRANSCRANIAL MAGNETIC STIMULATION” (PCT/US2008/073751), filed Aug. 20, 2008; “FOCUSING MAGNETIC FIELDS WITH ATTRACTOR MAGNETS AND CONCENTRATOR DEVICES” (PCT/US2008/075575) filed Sep. 8, 2008; “PITCH, ROLL, AND YAW MOTIONS FOR ELECTROMAGNET ARRAYS”(PCT/US2008/075583), filed Sep. 8, 2008; “FOCUSED MAGNETIC FIELDS” (PCT/US2008/075706), filed Sep. 9, 2008; “AUTOMATED MOVEMENT OF ELECTROMAGNETS TRACKING ECCENTRICITY OF THE HEAD” (PCT/US2008/075824), filed Sep. 10, 2008; “SYSTEMS AND METHODS FOR COOLING ELECTROMAGNETS FOR TRANSCRANIAL MAGNETIC STIMULATION” (PCT/US2008/077851), filed Sep. 26, 2008; “DISPLAY OF MODELED MAGNETIC FIELDS” (PCT/US2008/079378), filed Oct. 9, 2008; “INTRA-SESSION CONTROL OF TRANSCRANIAL MAGNETIC STIMULATION” (PCT/US2008/081048), filed Oct. 24, 2008; “TRANSCRANIAL MAGNETIC STIMULATION WITH PROTECTION OF MAGNET-ADJACENT STRUCTURES” (PCT/US2008/081307), filed Oct. 27, 2008; and “MONOPHASIC MULTI-COIL ARRAYS FOR TRANSCRANIAL MAGNETIC STIMULATION” (U.S. patent application Ser. No. 12/185,544), filed Aug. 4, 2008.